Preventing session.el messing up with geiser-doc--history

Session seems to be recovering the value of geiser-doc--history
badly (see issue #7 for @achitu's discoveries), and since it is, in
fact, not a good idea to save it anyway, we've added an eval-on-load
deregistering the variable from session's list.
NEWS update
Experiment setting history file coding system
Another not entirely satisfactory experiment with raw-text
Using session instead of session.el for you bytecompilers